Automatic system for collecting, weighing and releasing solid particles

ABSTRACT

The invention relates to an automatic system for collecting, weighing and releasing solid particles comprising a leak-tight weighing chamber (1) which houses a container (5), whereon the particles entering said chamber (1) via an inlet (4) come to rest. The weight of the container (5) is transmitted to a weighing sensor (7) which is also located inside the weighing chamber (1) but which does not come into contact with the walls of said chamber. Once the sample has been weighed, it is removed from the chamber through the injection of compressed air. The inventive system can be used directly to determine the mass flow of solid particles in pneumatic conveyor lines by means of the integration thereof in automatic sampling systems. For this purpose, a signal is emitted which can be integrated in flow regulation loops in order to control said flow.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] PCT Application ES01/00200; Filed May 22, 2001.

BACKGROUND OF THE INVENTION

[0002] 1. Field of Invention

[0003] There are numerous industrial processes which require the transport of solids through ducts using a carrier gas stream. Examples are the food industry, the pharmaceutical industry and, especially, combustion processes using solid fuel, such as power plants, cement factories and blast furnaces.

[0004] Although for gases and liquids there has traditionally been a wide variety of equipment for continuous measurement of the mass flow through ducts, in the case of pneumatically transported solids, it is only in recent years that promising technologies have been developed for determining this parameter.

[0005] The importance of measuring the mass flow of solids depends on the particular process involved. For coal-fired power stations this measurement is of vital importance from the point of view of the performance and operation of the plant and the emission of pollutants.

[0006] Most coal-fired power stations use direct fuel feed systems. In these systems, the coal or other solid fuel is pulverised in several mills, where a stream of hot air dries the coal and carries it through several ducts to the burners located in the boiler furnace. The number of burners fed by each mill varies from one installation to another, but there are generally 4-6 of them. The transport ducts from the mills to the burners are, in general, of different lengths and layouts, and with different types and numbers of bends and changes of direction. This difference in hydraulic resistance between different ducts, and possible asymmetries in the flow inside the mills or the classifiers causes significant imbalances in the supply of coal and air to each burner.

[0007] These imbalances have a negative influence on performance and on the generation of pollutants in this type of installation, in such a way that the existence of critical areas with an inappropriate air/coal ratio causes penalties with respect to these extremely important parameters. These imbalances can also generate areas of high reduction which cause a drop in the fusion point of the ash, and, consequently, increase the propensity of ash deposition on the boiler pipes (slagging). This causes significant losses due to heat transfer, and at times causes problems which make it necessary to shut down the plant, with the consequent loss of production.

[0008] A common practice in coal-fired power stations to reduce these imbalances is to add orifices of different diameters into the ducts to the burners, causing different pressure drops in order to equalise the hydraulic resistance of the different ducts.

[0009] The flow is balanced in each duct by means of measurements taken during tests using clean air, with no coal. Due to the peculiarities of the flow of biphasic mixtures, this balancing with clean air does not guarantee a balanced distribution of coal during normal operation of the mill. In addition, the imbalances in the flow of coal between the different ducts of a mill varies, in magnitude and direction, with mill operation, and so the above adjustment has a limited efficiency.

[0010] The environmental limitations of recent years have encouraged the use of low NO_(x) burners which minimise the generation of this pollutant by adjusting the stechiometry of the flame. The optimum operation of this type of burner is conditioned by a tight control of the air/coal ratio.

[0011] All of the foregoing underlines the need for providing these installations with control loops to regulate the supply of coal to the boiler. These control loops should be based on adjustable mechanisms, such as dampers, directional vanes, etc., as well as on reliable measurements of the flow of coal to each of the burners.

[0012] 2. Description of Related Prior Art

[0013] At present there exists a wide range of equipment for measuring the flow of pneumatically transported solids. This equipment falls into two large groups: extractive and non-extractive equipment, depending on whether samples are taken from the ducts.

[0014] Extractive equipment aspirates a sample of the solids carried in each duct. This sample, after being weighed and analysed in the laboratory, makes it possible to obtain the mass flow and the particle size distribution.

[0015] This equipment is based mainly on international standards developed for the sampling of pulverised coal. These standards, ASTM D197-87, ISO 9931 and the ESI Industry Standard, specify a method and the basic components of pulverised coal sampling equipment in circular ducts, with the ultimate aim of determining the flow of coal and the particle size distribution.

[0016] The ASTM D197-87 standard establishes the use of manual equipment, using a probe equipped with a single nozzle which is taken to different locations on two diameters at 90° to each other in a transversal section of the duct. The sample must be aspirated at the same velocity as the carrier air stream (isokinetic velocity). This aspiration velocity must be adjusted in order to obtain a true reading, since a lower velocity would give a sample with a lack of fines and a higher velocity would overestimate these.

[0017] The most commonly used method for sampling pneumatically transported solid particles is that specified by the ISO 9931 standard. This standard has been used as a reference by several manufacturers of equipment, such as M&W Asketeknik, GEE-EER, INERCO and Mission Instruments. These are all manual, with a rotating probe and four aspiration nozzles, each one of which sweeps the same area of the transversal section of the duct as it turns. The sample is extracted isokinetically by means of adjusting the suction provided by an ejector and is separated from the air flow by a cyclone.

[0018] This last method presents, in comparison to the previous one, a significant improvement as regards the representativity of the sample, and is less sensitive to the effects of particle segregation produced by disturbances in the flow.

[0019] Equipment based on the ESI Industry Standard uses null pressure probes which are manually positioned at different places on the transversal section of the duct, aspirating at each point at isokinetic velocity. Examples of commercial equipment using this method are the SMG-10 and AKOMA systems. The advantage of this method, in comparison to the others, is that it aspirates isokinetically at all points of the section, and its main disadvantage is the length of time needed for the sampling.

[0020] Other equipment, based on the methods described above, has also been developed to automate sampling. One example of this type of equipment is the APFS developed by M&W Asketeknik (patent W09810266). This equipment is based on the ISO 9931 standard, with regard to the shape of the probe and aspiration nozzles, as well as the component parts and the operating principle. The innovation it includes is the installation in each duct of probes which can be inserted and retracted from the duct automatically using a rack and pinion type mechanism actuated by an electric motor.

[0021] As an advanced version of the above equipment, M&W Asketeknik has developed the ACFM-2100, which, using the same probe and the same actuating mechanism, incorporates in a control unit a cyclone for separating particles, connected to a continuous system for inferring the amount of sample extracted during each sampling. This system consists of a cylindrical chamber which during sampling is filled to a certain level. An optical sensor detects the level of particles in the chamber and therefore the volume of the sample extracted. The flow of solids in the duct is inferred from the apparent density of the particles.

[0022] The disadvantage of this system is that it produces results by indirect measurement of the weight of the particles by determining the volume of the sample. This may give rise to significant errors in the case of changes in the density of the solid or distribution of the size of particles, requiring continuous adjustment of the equipment to guarantee the quality of the measurement.

[0023] The other group of equipment for the measurement of the mass flow of particles is made up of non-extractive systems, which have developed greatly in recent years. These systems are based on the measurement of some physical property in the biphasic flow related to the concentration of the particles. In addition to methods based on electrostatic, acoustic and mechanical methods, all the regions of the electromagnetic spectrum, from gamma rays to microwaves, have been used for the development of these sensors.

[0024] These measurement systems have the disadvantage of not being very precise and accurate, due to the fact that they are sensitive to parameters such as the humidity content of the mixture, the size of particles and the phenomena of segregation in the ducts. In addition, they require periodic calibration using comparative measurements carried out using extractive methods, and do not allow for the obtention of a sample for later laboratory analysis.

[0025] Of all the documents analysed, those which most resemble the device in question are systems for weighing bulk materials, either for continuous flow measurement or for packaging or bagging. Examples are U.S. patents U.S. Pat. No. 4,513,830 (Teccon) and U.S. Pat. No. 5,121,638 (Buehler). However, these devices, in addition to having other aims, lack, among others, the characteristics related to the particle expulsion system.

[0026] Within the specific field of particle transport in a gas stream, there is U.S. patent U.S. Pat. No. 4,758,118 (Rachner & Schott), although the aim of the procedure and the device described is not so much the measurement of the flow of an already established stream, but the previous stage of introduction of a measured amount of particle material in a gas stream to make up a particular mass flow. For this purpose, the weight of the pulverised material is measured using a container for weighing which uses electrical load cells on which that container rests (column 5, lines 8 to 17). The description of the weighing container, however, is not given in more detail, and so it can be considered that it does not affect the novelty of the device as described in claims 1 to 7.

[0027] U.S. patent U.S. Pat. No. 4,490,077 (Nippon Kohan KK / Sankyo Dengyo KK), in the same sector, presents a similar case, where load cells are also used for continuous measurement of particles in a container. Here, again, the brief description of the weighing device allows us to conclude that it is not relevant for the application analysed. Other patents have been recovered with similar aims (dosed feed of particles to gas streams by weighing), but none of these concentrates on the weighing device. In some of these, indeed, such as U.S. Pat. No. 4,669,921 (Charbonnages de France), it is specified that the gravimetric measuring device used is one available on the market (column 3, lines 11 to 17).

[0028] Another document related to this subject is U.S. patent U.S. Pat. No. 4,838,738 (Shell Oil Co.), where the mass flow of a mixture of solid particles and gas supplied to a reactor is controlled by determining the weight of the mixture in a container. In this patent also, although a weighing device is used, the general layout is different.

[0029] All of this means that at present none of these technologies has reached a sufficiently robust state for large-scale industrial implementation.

BRIEF SUMMARY OF THE INVENTION

[0030] This invention is an automatic system for the capture, weighing and expulsion of solid particles which reach the system after being separated from a gas stream by means of a cyclone, or by gravity, through devices such as hoppers, funnels, storage chambers, etc. This system can be applied directly to the continuous measurement of the flow of particles suspended in a stream of carrier gas.

[0031] With regard to direct applications, the system can be used jointly with any isokinetic solid particle sampling system in pneumatic carrier ducts. These sampling systems may be based on any of the existing standards for the sampling of these particles, ASTM D197-87, ISO 9931 or ESI Industry Standard, and can be applied to any system which supposes automation of the equipment and procedures described in these standards.

[0032] One example of this application would be in coal-fired power stations, in which the coal is pulverised in mills and then transported pneumatically through several ducts to the burners located in the boiler furnace.

[0033] There are several international standards for obtaining representative samples from these ducts, which make it possible to obtain, based on the weight of the sample, the flow of coal fed to the boiler through each one, and the size distribution of the particles or other parameters which may be of interest for the operation and state of the mills.

[0034] The system to be patented may be integrated in automatic particle sampling equipment based on any of these international standards, making up a system which makes it possible to carry out continuously measurement of the mass flow of solids transported pneumatically through a transport duct.

[0035] The measurements obtained may also be integrated in flow regulation systems, either in the ducts themselves or in the mills, constituting control loops which make it possible to balance the supply of solids through different ducts and provide a uniform feed to downstream equipment. In the case of coal-fired power stations, the imbalances in the supply of fuel to the boiler have a negative effect on the performance of the unit, as well as on the generation of nitrogen oxides and localised slagging.

[0036] The system makes it possible to obtain automatically a precise weighing of the sample extracted, which makes it possible to determine the mass flow of said particles in the transport stream, depending on: the number and the area of sampling nozzles, the velocity of aspiration of the sample, the area of the transport duct and the time taken for sampling.

[0037] As well as providing the flow measurement, with this system it is possible to evacuate the sample for a new weighing, returning it to the duct from which it was taken or transporting it to a container for further laboratory analysis.

[0038] Therefore, the system constitutes a continuous measurement system of mass flow of solid particles in pneumatic carrier ducts.

[0039] The system consists basically of a chamber with an orifice in the top part through which the particles fall. Inside this chamber there is a container for collecting the solid particles which enter the chamber. The container rests freely on a weighing sensor without contacting the walls of the chamber, since any contact could transmit part of the weight of the particles to the walls with the resulting error in measurement.

[0040] The weighing chamber has one or more ducts for the entry of compressed air. The compressed air enables the system to evacuate the sample through several exit ducts, and so, without subjecting the sensor to overpressure which may damage it, the total extraction of the particles from the container is guaranteed and at the same time the weighing chamber is cleaned, avoiding the accumulation of particles in the interior of the chamber which could affect future measurements.

[0041] The system described gives a direct measurement of the mass of the particles collected, which is an advantage with respect to those at present existing and installed in automatic isokinetic sampling equipment, that are based on measurements of the volume of the sample collected using optical sensors. This measurement is related directly to the mass flow of particles in the duct, independently of the nature of those particles, their size distribution and their apparent density. Therefore there is no need for additional adjustments to infer the flow of particles when their characteristics change.

[0042] With respect to continuous measuring systems of the non-extractive type mentioned in the previous section, the system described has the following advantages:

[0043] It allows a sample of particles to be obtained. This sample may be taken to the laboratory for the determination of important parameters such as humidity and the particle size distribution, or physical and chemical characteristics.

[0044] It provides more accurate results since it is based on direct measurement of the mass collected according to a reference method.

[0045] It needs no calibration using comparative measurements. Non-extractive measuring equipment carries out indirect measuring which is related to the particle flow through laborious calibrations using manual reference systems. In this aspect, the system described, associated with automatic portable sampling equipment, makes it possible the automatic execution of calibration routines for these measuring systems.

[0046] The system requires only the calibration of the weighing sensor, which may be carried out during periodic maintenance activities, using standard reference masses.

[0047] This invention can be applied to any system of pneumatic transport of solids. In the particular case of pulverised coal power stations, it can be used for the detection of imbalances in coal feeds among the different burners of the boiler. It also provides a signal which can be integrated in the automatic control loops of the mills to provide optimum adjustment of the operating conditions of the unit, as well as in the specific solid flow control loops based on regulating systems such as dampers, flow deflectors or directional vanes installed within the ducts or inside the mills or classifiers.

BRIEF DESCRIPTION OF THE DRAWINGS

[0048] For a better understanding of the present invention, drawings are attached showing, as examples only, some variations on the system layout.

[0049]FIG. 1 presents a diagram of the automatic capture, weighing and expulsion system, showing the weighing chamber (1), inside which there is an inner container (5) that, without contacting the walls of the outer container (2) collects the solid particles which enter through the weighing chamber sample inlet duct (4). This container (5) rests on a platform (6) which is supported on a weighing sensor (7) connected to a transmitter (11) which generates a signal (for example 4-20 ma) depending on the weight resting on the sensor. The figure shows the compressed air inlet ducts (14) and the outlet ducts (15) through which the sample is evacuated once it has been weighed.

[0050]FIG. 2 shows the automatic capture, weighing and expulsion system integrated in an automatic solid particle sampling system based on the ISO 9931 standard. The joint system constitutes a continuous measurement system of the flow of solids in pneumatic carrier ducts. The figure shows a rotating probe (19) which may be inserted or retracted from the duct (18) by means of a pneumatic cylinder (23).

[0051] The sample aspirated circulates through a cyclone (28) where the particles are separated from the gas stream, falling by gravity into the weighing chamber (1). Once the sample has been weighed it is evacuated into a container for laboratory analysis or returned to the duct (18).

[0052]FIG. 3 shows a detail of the extraction system of a possible automatic solid particle sampling system. The figure shows a cylindrical tube (20) with four 5 mm holes through which the sample is extracted. This tube (20) is a variant of the nozzles specified in the ISO 9931 standard, and has the advantage that it is less likely to be jammed by solids. The tube rotation mechanism (20), which uses an angular gear (21), is also shown, as well as the cylindrical chamber (25) into which the probe is retracted when sampling is not necessary.

[0053]FIG. 4 shows the basic application of the automatic capture, weighing and expulsion system for the determination of the size distribution of the weighed sample. The expulsion system transports the sample, using a known transport stream, to two cyclones in series (34) and (35), each of which is connected to an additional weighing system (36) and (37). Depending on the design of the cyclones (34) and (35) and on the fraction of the sample separated in each of them, the size distribution of the original sample can be inferred.

FIGURE LEGEND

[0054]1. Weighing Chamber

[0055]2. Weighing Chamber Outer Container

[0056]3. Weighing Chamber Lid

[0057]4. Weighing Chamber Sample Inlet Duct

[0058]5. Weighing Chamber Inner Container

[0059]6. Platform

[0060]7. Weighing Sensor (or Load Cell)

[0061]8. Cylindrical Concentric Lip

[0062]9. Weighing Chamber Inlet Ball Valve

[0063]10. Pneumatic Actuator

[0064]11. Transmitter

[0065]12. Seal

[0066]13. Stops

[0067]14. Compressed Air Inlet Duct

[0068]15. Weighing Chamber Outlet Duct

[0069]16. Weighing Chamber Outlet Ball Valve

[0070]17. Weighing Chamber Outlet Valve Actuator

[0071]18. Process Duct/Pipe

[0072]19. Rotating Probe

[0073]20. Sample Extraction Tube

[0074]21. Angular Gear

[0075]22. Electric Motor

[0076]23. Pneumatic Cylinder

[0077]24. Bracket

[0078]25. Cylindrical chamber

[0079]26. Bypass

[0080]27. Control Unit

[0081]28. Cyclone

[0082]29. Air Flow Regulator

[0083]30. Weighing Chamber Sample Expulsion Valve

[0084]31. Cyclone

[0085]32. Weighing Chamber Sample Expulsion Valve

[0086]33. Weighing Chamber

[0087]34. Cyclone

[0088]35. Cyclone

[0089]36. Weighing Chamber

[0090]37. Weighing Chamber

[0091]38. Lab Sample Container

[0092]39. Pneumatic Line For Probe Cleaning

[0093]40. Probe Sample Valve

[0094]41. Pressure Relief Valve

DESCRIPTION OF A PREFERRED EMBODIMENT

[0095] The solid particle capture, weighing, and expulsion system consists of a weighing chamber (1) made up of a cylindrical container and a screw lid (3). The lid (3) has a particle entrance duct (4); the particles enter the chamber (1) by gravity and are collected in a container (5) which is also cylindrical with an exterior diameter 2 millimetres less than the internal wall of the container (2). The container (5) has a conical base to facilitate the elimination of the particles, and is joined to a platform (6) attached by screws to a load cell (7) on the base of the container (2) and supported above it by two screws. In this way the weight of the sample is transmitted to the load cell (7) as the sample is collected.

[0096] The arrangement of the container (5) inside the chamber (1) composed of parts (2) and (3) is concentric to these, and so between the container (2) and the container (5) there is a 1 millimetre space in the form of a circular corona. The height of the container (5) is such that on screwing the container (2) tightly onto the lid (3) there is a 1 millimetre space between the inside of the lid and the top of the container (5), in order to facilitate the entry of air for the expulsion of the particles.

[0097] The lid (3) also has a cylindrical concentric lip (8) of a diameter such that between its outside face and the inside surface of the container (5) there is a space of approximately 1 millimetre in the form of a circular corona.

[0098] The particle inlet duct (4) is connected to a ball valve (9) activated by a pneumatic actuator (10) driven by a 3-way electrovalve. This ball valve (9) opens during the entry of particles and, on closing, seals the weighing chamber (1).

[0099] The load cell (7) is connected to a transmitter (11) which generates a signal of between 4-20 mA proportional to the mass deposited on the cell (7). This transmitter (11) is outside the weighing system, and so the connection cable passes through the container (2) through a seal (12) which ensures that the weighing chamber (1) remains airtight.

[0100] The load cell (7) is protected against excess pressure by two physical stops (13) located on a threaded rod on the base of the container (2) which limit the maximum flexing of the cell (7).

[0101] The base of the container (2) has two ducts for the entry of compressed air (14) to expel the sample once it has been weighed. The lid (3) also has two ducts for the expulsion of the sample (15) by the compressed air. These ducts (15) enter the weighing chamber (1) as far as the conical part of the container (5), and are connected to a second ball valve (16) which is pneumatically activated and is closed during the sample collection cycle, to avoid the entry of air into the weighing chamber (1).

[0102] The operating sequence of the solid particle capture, weighing and expulsion system described above is as follows:

[0103] During the collection of the sample, the ball valve (9) remains open so that the particles are collected in the container (5). The maze formed by parts (2), (3) and (5) means that it is unlikely for particles to reach the load cell (7) during the collection of the sample.

[0104] Once the particles have been captured, the ball valve (9) closes, and the transmitter (11) connected to the load cell (7) takes a reading.

[0105] Then the sample is expelled by means of compressed air injected through the ducts (14) located in the base of the container (2), after the valve (16) is opened. This air reaches the container (5) through the maze made up of parts (2), (3) and (5), and raises the mass of particles and generates a vortex which facilitates its expulsion through the ducts (15).

[0106] The expulsion air should be previously filtered to eliminate oils and drops of condensed water. In addition to serving as the medium for expulsion of the particles, this air also cleans any particles which may be left on the walls of the container (5) and dries any humidity from the particles possibly condensed on the walls.

[0107] The injection of compressed air through the lower part of the system also avoids the deposit of particles in the circular corona between the container (2) and the container (5). This deposit of particles would create a certain contact between these parts, which would mean that the weight of the particles would not be applied in its entirety to the load cell (7), and would thus give a low weight reading. The expulsion system also avoids the accumulation of dust on the load cell (7) which could damage it.

[0108] A direct application of the system of capture, weighing and expulsion described above is its integration in automatic isokinetic sampling systems of pulverised coal in the transport ducts to the burners in power stations. FIG. 2 shows, as an example, an integrated system for determining the mass flow of coal in a duct.

[0109] This system consists of a rotating probe (19) similar to that specified in the ISO 9931 standard, except that it uses a cylindrical tube (20) with four 5 mm holes for the extraction of the sample, which are opposite the stream of air and coal and are located as indicated in the standard with respect to the axis of the duct, depending on its diameter. This cylindrical tube (20) rotates around the axis of the duct (18), by means of an angular gear (21) actuated by an electric motor (22).

[0110] The probe (19) can be inserted into and retracted from the duct (18) by means of a pneumatic cylinder (23) which is supported on a bracket (24) fastened onto the duct (18).

[0111] When the probe (19) is retracted, the cylindrical tube (20) remains in the cylindrical chamber (25). Before the beginning of a sampling sequence, air is provided via a pressurized air line (39) through the probe sample valve (40) to the probe in order to clear possible obstructions from the sample intake holes, and to clean the chamber (25) of possible coal deposits.

[0112] To make this cleaning flow possible, the chamber (25) has a by-pass line (26) which connects it to the inside of the duct (18) as shown in FIG. 3. There is no flow from the duct (18) to the chamber (25) through this by-pass (26), since both are at the same pressure, the latter being sealed from the surrounding air. Flow is initiated from the chamber (25) to the duct (18) when the probe sample valve is opened and pressurized air is admitted for cleaning from the control unit (27).

[0113] The probe (19) is connected to the control unit (27) where there is a pneumatic system and the necessary instrumentation, as indicated in the ISO 9931 standard, for the production of the suction required to extract the sample. The aspiration velocity of the sample is calculated from data on the operation of the mills, such as the transport air flow and the temperature and pressure of the air-coal mixture in the ducts. This data, which is monitored in the power station control room, is processed by a PC which calculates the necessary suction and sends a signal via a communications cable to the control unit (27). This control unit establishes and controls the calculated aspiration velocity, and the overall operation of the equipment. A relief valve (41) is provided for the pneumatic systems.

[0114] The sample from the probe (19) is transported via duct or pipe to a high efficiency cyclone (28) which separates the particles of coal from the sample extracted. The separated particles fall by gravity into the capture, weighing and expulsion system. Once the sample has been taken, the control unit (27) closes the pneumatic valve (9) and weighs the sample collected. The weight signal generated by the transmitter (11) is sent to the control unit (27) for processing. Once this signal is received, the sample is expelled by the injection of filtered compressed air from the control unit (27). The expulsion air flow can be regulated by the regulator (29).

[0115] The sample expelled may be returned to the transport duct (18) by opening the pneumatic valve (30) during the expulsion phase, or taken to a cyclone (31) to be collected at the lab sample station (38) for analysis in a laboratory, in which case the pneumatic valve (32) would be opened.

[0116] An additional application of the automatic capture, weighing and expulsion system, it can be used to automatically determine the size distribution of the sample collected. FIG. 4 shows the basic layout for the determination of this size distribution. Once the sample has been collected and weighed in an initial capture, weighing and expulsion system (33), it is transported by an expulsion air stream, whose design flow is established by the flow regulator (29), to two cyclones (34) and (35) connected in series. These two cyclones are designed depending on the design air flow for expulsion, to produce two substantial fractions of the original samples. These two fractions are quantified in two capture, weighing and expulsion systems (36) and (37). Depending on the weights and the efficiency of each cyclone to capture the particles contained in certain predetermined ranges of sizes, the percentage of particles from the original sample with sizes in each of those ranges may be inferred. 

1. Autoniatic system for the capture, weighing and expulsion of solid particles which may enter said system after being separated from a transport gas stream by cyclones or by gravity, by means of devices such as hoppers, funnels or storage chambers consisting of the following elements: A) an airtight chamber (1) which the particles reach through one entrance duet (4) having inside a container (5) (where the particles arc stored) which without contacting the inner walls of the airtight chamber (1) transmit the weight of the sample to a weighing sensor (7); B) a ball valve (9) with pneumatic operation (10) connected to the airtight chamber (1) which is closed during the cycles of expulsion of the sample; C) a transmitter (11) which generates a signal of 4-20 mA depending on the weight placed over the weighing sensor (7); D) a system for expulsion of the sample based on the injection of compressed air into the weighing chamber (1) which transports the particles deposited in the container (5) through one or more expulsion ducts (15).
 2. System in accordance with claim 1 characterized by the fact that the weighing chamber (1) is formed by a cylindrical container (2) closed at the top by a lid (3) with the entrance duct for the particles (4) and a cylindrical lip (8) concentric with the container (2) and the container (5), being the dimensions of all these components such that there is a small lateral space of less than 1 mm to the cylindrical lip (8) and the container (2), forming a pneumatic maze which makes difficult for the particles to reach the weighing sensor (7).
 3. System according with claims 1 and 2 characterized by the fact that the container (5) is stored in its upper part to ease the entrance of air for the elimination of the particles during the cycles of expulsion, with a conical shape at its base and a height such that when the set is mounted, the upper edge has a space of less than 1 mm to the lid (3).
 4. System in accordance with claims 1, 2 or 3 characterized by the fact that the weighing sensor (7) is a load cell located inside the weighing chamber (1) and is supported above the base of the container (2) by screws and has a protection system against access efforts consisting of two physical stops (13) placed on a threaded rod also supported on the base of the container (2) which limit the maximum deflection the load cell is able to suffer.
 5. System in accordance with claims 1, 2, 3 or 4 characterized by the fact that the transmitter (11) is located outside the weighing chamber (1) being connected to the weighing sensor (7) by means of a cable which passes through the container (2) through a seal (12) in such a way as to ensure the air tightness of the weighing chamber (1).
 6. System in accordance with claims 1, 2, 3, 4 or 5 characterized by the fact that the expulsion system of the sample consists of the injection of filtered compressed air, after closing the pneumatic valve (9), through one or more ducts on the base of the container (2) of the weighing chamber (1) reaching said expulsion air the interior of the container (5) through the maze delimited by the lid (3), the container (2) and the container (5), thus creating a vortex which lifts and transports the particles outside the weighing chamber (1) through one or more expulsion ducts (15) located on the lid (3).
 7. System in accordance with claim 6 characterized by the fact that the expulsion ducts (15) for the sample located on die lid (3) enter the weighing chamber (1) to a depth such as to reach the conical part of the container (5) being their flow through restricted during the collection of the sample by means of the closing of a ball valve pneumatically operated (16), to which they are connected.
 8. System in accordance with claims 1 to 7 where there is a control system based on a programmable device, or computer which governs: A) the opening and closings of the pneumatic valve (9) of access of the sample; B) the injection of compressed air for the expulsion of the sample; C) the opening and closing of the pneumatic valve (16) connected to the expulsion ducts of the sample (15); D) the transmitter (11) signal of tie weighing sensor (7), establishing in case of that the total expulsion has not been completed, new cycles of blowing.
 9. System in accordance with any of the claims 1 to 8 integrated in automatic sampling systems based on international standards such as ASTM D197-87, ISO 9931 and ESI Industry Standard, for the direct determination of the mass flow of solid particles in pneumatic transport ducts (18), being said system connected to a cyclone (28) which captures the particles during the aspiration of the sample and offering the possibility of expelling the weighed sample to the transport duct (18) or a second cyclone (31) for the collection of the sample for laboratory analysis.
 10. System in accordance with claim 9 characterized by the fact that the signal of weighing obtained is related to the mass flow of particles in the duct and is implemented in regulatory loops of flow for the control of said flow.
 11. System in accordance with claims 9 or 10 applied to the calibration of other continuous measuring systems of the mass flow of particles installed in pneumatic transport ducts.
 12. System in accordance with claims 1 to 11 applied to the continuous determination of the size distribution of the sample obtained by means of the transport of said sample by a design expulsion air stream to one or more cyclones (34), (35), connected in series, of which the fractional efficiency curve is known, and which are designed to produce two substantial fractions of the original sample which weights, determined in two automatic capture, weighing and expulsion systems, allow to determine the size distribution of the original sample.
 13. System in accordance with claims 1 to 8 applied to any system involving the extraction of a sample of particles suspended in a transport gas stream in order to obtain an automatic weighing of the obtained sample.
 14. System according with claims 1 to 8 applied to any process in which it is required to know in an automatic and precise manner the weight of successive samples of solid particles which reach the weighing chamber (1) by any procedure, for further transport to some storage system or for incorporation in some process as reactive or as constitutive element of a final product. 